1. Field of Invention
The present invention relates to a passive infrared sensor that detects the presence of an intruder in a security area by receiving the infrared light that the intruder emits, and in particular to a passive infrared sensor that can detect sabotage of the operation of the passive infrared sensor and to an obstacle detection system used therein.
2. Conventional Art
A passive infrared sensor is configured to receive infrared light from an intruder in a detection area set in a security area and to detect the presence of the intruder from the difference in temperature between the intruder's body and the surrounding area. An infrared light receiving window for introducing the light of the detection area is disposed in the passive infrared sensor, but when there is sabotage, such as when the outer side of the light receiving window has been deliberately covered with some kind of light blocking object, the passive infrared sensor loses its detection function. When the passive infrared sensor loses its detection function, alarm signals are not outputted even if there is an illegal intruder. Actual sabotage may include a case where, during the time that the passive infrared sensor is inoperative when the coming and going of people into and out of the room disposed with the passive infrared sensor is high, someone deliberately sprays transparent paint that does not transmit far-infrared light, or adheres adhesive tape, on the front surface of the cover of the security sensor, so that the passive infrared sensor becomes unable to detect the presence of a human body, and an intruder intrudes into the room during the time that the passive infrared sensor is operative when people are no longer entering and leaving the room.
A security sensor disposed with a radiant energy detection apparatus that detects the presence of a light blocking object interfering with the detection function has been proposed (e.g., see Japanese Patent Application Laid-Open Publication (JP-A) No. 2-287278). This radiant energy detection apparatus is disposed with a light emitting element, which emits near-infrared light or visible light towards the inner surface of a portion of a cover of the security sensor through which far-infrared light from a human body passes, and a light receiving element, which receives the reflected light of the near-infrared light from the inner surface of the cover. The radiant energy detection apparatus is configured to detect the presence of an obstructive object on the outer surface of the cover by detecting an increase in the amount of incident light at the light receiving element resulting from the light reflected from the obstructive object applied to the outer surface of the cover being added to the light reflected from the inner surface of the cover.
FIGS. 3(a) and 3(b) are schematic views describing the operating principle of a passive infrared sensor 20 applying this prior art. FIG. 3(a) shows an ordinary state where an obstructive object 8 is not present, and FIG. 3(b) shows a state where the obstructive object 8 is approaching.
As shown in FIGS. 3(a) and 3(b), a lens 4 is disposed in a light receiving window formed in the center of the front surface (the left side in the drawings) of a box-like case 22 of the passive infrared sensor 20. Infrared light from a detection area A0 is guided by the lens 4 to a passive infrared light receiving sensor 5 disposed inside the case 22 in the center of the back (the right side in the drawings) of the case 22. Moreover, an infrared light emitting diode 6 is disposed in the vicinity of the lens 4 at an upper portion inside the case 22 and configured to emit obstructive object detection-use infrared light through the lens 4 and diagonally downward to the outside of the case 22. An infrared light receiving diode 7 is horizontally disposed in the vicinity of the lens 4 at a lower portion inside the case 22 and configured to receive the infrared light coming from the outside of the case 22 and transmitted through the lens 4.
As shown in FIG. 3(a), in an ordinary state where the obstructive object 8 is not present, the infrared light L1 emitted in the front direction of the infrared light emitting diode 6 proceeds without being obstructed. Thus, the infrared light L1f reflected by some kind of object ordinarily does not return to the infrared light receiving diode 7. However, the infrared light L2 which is the part of the emitted infrared light inside the case 22 within the projection angle of the infrared light emitting diode 6 is reflected by the inner surface of the lens 4, and the reflected infrared light L2a reaches the infrared light receiving diode 7. The amount of light received by the infrared light receiving diode 7 in this case is an intermediate value (reference received-light amount) corresponding to the ordinary state where the obstructive object 8 is not present.
As shown in FIG. 3(b), when the obstructive object 8 approaches the passive infrared sensor 20, the infrared light L1 emitted in the front direction of the infrared light emitting diode 6 is reflected by the surface of the obstructive object 8, and the reflected infrared light L1f here reaches the infrared light receiving diode 7. For this reason, the amount of infrared light received by the infrared light receiving diode 7 becomes the sum of the infrared light L2a and the infrared light L1f, and becomes larger than the intermediate value corresponding to the ordinary state where the obstructive object 8 is not present. In this manner, the passive infrared sensor 20 can detect the approach and/or presence of the obstructive object 8 using the change in the amount of infrared light received by the infrared light receiving diode 7. However, because there is little infrared light L1f in a case where the obstructive object 8 is a light absorber such as black cloth, the amount of infrared light received by the infrared light receiving diode 7 does not change that much. For this reason, there are cases where reliable detection is not possible depending on the type of obstructive object 8.
As other prior art, an infrared human body detection apparatus has also been proposed which, when a light blocking object resulting from sabotage has been placed over the light receiving window and when a light blocking object has been placed away from the light receiving window, immediately detects the light blocking object even if it is a light absorber such as black cloth or a black plate and outputs a detection signal (e.g., see JP-A No. 7-174622). This infrared human body detection apparatus includes a sensor that receives, through the light receiving window, the infrared light that a human body emits and detects the presence of a human body with an electrical signal of the sensor. The infrared human body detection apparatus also includes a light emitting element that emits infrared light from the outer side of the light receiving window, a light receiving element disposed at the inner side of the light receiving window, and an obstructive object detection optical path that guides some of the light emitted by the light emitting element to the light receiving element. According to this infrared human body detection apparatus, when a blocking object is adhered to and covers the light receiving window, the amount of light made incident at the light receiver is reduced, and the fact that the detection apparatus has been sabotaged is detected from the change in the amount of received light. Also, when a blocking object has been placed away from the light receiving window, the light reflected by the blocking object is made incident at the light receiver in addition to the light made incident at the light receiver from the light emitter when there is no obstructive object. Thus, the amount of incident light at the light receiver increases, and the fact that the detection apparatus has been sabotaged is detected from the change in the amount of incident light.
However, in the above prior art, it has been mainly assumed that the passive infrared sensor is disposed indoors. When the passive infrared sensor is disposed outdoors, the light receiving element that receives the obstructive object detection-use infrared light is affected by strong ambient light such as sunlight, and there is the possibility for the passive infrared sensor to become unable to exhibit a sufficient obstructive object detecting capability or for the passive infrared sensor to malfunction. When such a device is disposed outdoors, sometimes frost or the like adheres to the lens due to a radiation cooling phenomenon or the like during cold periods, and sometimes the obstructive object detecting capability drops due to some of the infrared light from the detection area not reaching the passive infrared light receiving sensor.